The present invention relates to a method of determining the color conversion characteristic of an image display device, such as a monitor, which displays color images, and which has a color conversion means converting, on a pixel-by-pixel basis, image data, referring to the color conversion characteristic. The invention also relates to an image display device having a color conversion means using the color conversion characteristic determined by such a color conversion characteristic determining method.
A conventional method of determining color conversion characteristic in an image display device is first described with reference to FIG. 10. FIG. 10 shows the configuration of a device using an image adjustment method disclosed in Japanese Patent Kokai Publication No. H5-48885. In FIG. 10, reference numeral 104 denotes a keyboard, 105 denotes a mouse, 106 denotes an input means, 107 denotes a controller, 108 denotes an input circuit, 109 denotes a memory, 110 denotes a CPU, 111 denotes an output circuit, 112 denotes an image display unit, 113 denotes an original image, 114a to 114h denote processed images, 115 denotes set parameters, and 116 denotes a hardcopy device. The keyboard 104 and the mouse 105 are examples of input means 106. The controller 107 comprises the input circuit 108 connected to the input means 106, the memory 109, the CPU 110, and the output circuit 111. The image display unit 112 is driven by the output circuit 111.
The illustrated color conversion characteristic determining method simulates the operation of the hardcopy device, and displays the image output from the hardcopy device on the image display device, and permits determination of the optimum color conversion characteristic for the hardcopy device, while observing the simulated images displayed on the image display device. The conventional method relates to a hardcopy device, but the concept of the color conversion characteristic determining method can be applied to image display devices. The operation of the color conversion characteristic determining method of FIG. 10 will next be described.
The memory 109 stores a color conversion simulation program. The CPU 110 executes the program stored in the memory 109. First, image data used for the color conversion simulation is input. The input image data is displayed on the screen of the image display unit 112. Then, the specific manner of processing is input by the use of the input means 106, and the color conversion simulated by the hardcopy device 116 is performed on the input image data, according to the designated manner of processing. The color-converted, processed images 114a to 114h are displayed on the screen of the image display unit 112, together with the original image 113. When the parameters for the color conversion are changed in multiple steps, the processed images are displayed together, arranged in an array. By selecting the processed image which is closest to the original image 113, the optimum parameters, i.e., the optimum conversion characteristic can be determined. When a plurality of color conversion parameters are determined, the operation similar to that described above is repeated for each of the parameters to determine the value of each of the parameters, in turn. The-thus determined color conversion parameters are sent to the hardcopy device.
The above color conversion characteristic determining method simulates the color conversion according to the specific manner of processing, designated by the input means 106, and permits the selection of the optimum image among the plurality of processed images, to thereby determine the color conversion parameters, i.e., the color conversion characteristic, so that it has an advantage that the operator can determine the color conversion parameters, i.e., the color conversion characteristic easily.
However, because the color conversion characteristic is determined according to the visual perception of the operator, it is not suitable for the determination of the color conversion characteristic for bringing the color reproduction closer to that of the standard, such as the sRGB standard, or NTSC standard. Moreover, the operator needs to have a clear idea of the image in the target color reproduction. If the target color reproduction is given in terms of numerical values, such as the chromaticity of the three primary colors, or the chromaticity of the white, then the operator cannot select the suitable one from among those obtained by simulation.
If the target color reproduction is given only in terms of numerical values, such as the chromaticity of the three primary colors, or the chromaticity of white, the method shown in FIG. 11 may be used. FIG. 11 shows a color conversion characteristic determining method which can be applied to an image display device including a color conversion characteristic holding means holding the color conversion characteristic, a color conversion means converting, on a pixel-by-pixel basis, the image data referring to the color conversion characteristic, and an image display means displaying the image data obtained by the color conversion at the color conversion means. This method is described next.
First, a color conversion characteristic candidate which is a candidate for the color conversion characteristic ultimately adopted is selected. Then, the color conversion characteristic candidate is set in the color conversion characteristic holding means. Then, tristimulus values according to the XYZ color system are measured for the colors displayed on the image display means, for each of a predetermined number of the image data (data having the same color throughout the screen). On the other hand, tristimulus values in the target color reproduction for the corresponding image data are calculated. The tristimulus values obtained by measurement and the tristimulus values obtained by calculation are compared. If the result of the comparison indicates xe2x80x9cNG (the predetermined condition is not satisfied)xe2x80x9d a new color conversion characteristic candidate is selected, and set in the color conversion characteristic holding means. If the result of the comparison indicates xe2x80x9cOK (the predetermined condition is satisfied)xe2x80x9d, then the color conversion characteristic candidate being used is output as the color conversion characteristic, and the processing for determining the color conversion characteristic is terminated.
For the measurement of the tristimulus values of the colors displayed on the image display means, it takes several seconds for each color. In the case of a high-precision measurement, it takes one minute or longer for each color. If the measurement is performed for each of 60 kinds (60 colors) of image data, and the tristimulus values are measured for each color, and it takes 10 seconds for each color, then it takes 10 minutes for the 60 colors. If the selection of the color conversion characteristic candidate through the comparison of the tristimulus values are repeated 50 times, the total time required is 500 minutes. The total time required for the determination is the sum of the time required for the measurement, and the time required for other works. When a high-precision measurement is performed, or when it is necessary to measure the tristimulus values of the colors displayed for a greater number of image data, or if it is necessary to repeat the selection of the color conversion characteristic candidate through the comparison of the tristimulus values a greater number of times, then even a longer time is required.
Thus, in the method in which a color conversion characteristic candidate is set in the color conversion characteristic holding means, and the tristimulus values of displayed colors are measured, it takes a long time for the determination of the color conversion characteristic, including the time for measurement of the tristimulus values of displayed colors.
The present invention has been made to solve the problems described above, and its object is to provide a color conversion characteristic determining method by which it is possible to determine the color conversion characteristic for bringing the color reproduction close to the ideal color reproduction according to the standard, and by which it is possible to determine the color conversion characteristic in a short time.
Another object of the invention is to provide an image display device using the color conversion characteristic determined by the above-mentioned color conversion characteristic determining method.
According to one aspect of the invention, there is provided a color conversion characteristic determining method for use with an image display device including a color conversion characteristic holding means holding a color conversion characteristic; a color conversion means converting, on a pixel-by-pixel basis, an image data represented by three or more colors, referring to said color conversion characteristic; and an image display means performing display based on the image data color-converted by said color conversion means; said method comprising the steps of:
(a) setting a conversion characteristic candidate, which is a candidate of said color conversion characteristic;
(b) generating sets of predetermined input image data;
(c) determining, using the color conversion characteristic candidate that has been set, predicted color-converted data, which are predicted values of the data output from said color conversion means when said input image data are input;
(d) determining predicted tristimulus values, which are predicted values of tristimulus values of the colors that are displayed on said image display means when said predicted color-converted data are supplied; and
(e) evaluating said predicted tristimulus values;
wherein said steps (c), (d) and (e) are repeated after determining a new color conversion characteristic candidate when said predicted tristimulus values are not found to satisfy a predetermined condition as a result of the evaluation of said predicted tristimulus values, and
said color conversion characteristic candidate then being used is adopted as said color conversion characteristic, when said predicted tristimulus values are found to satisfy the predetermined condition.
With the above arrangement, the tristimulus values of colors are determined by prediction calculation, so that it is not necessary to measure the tristimulus values of colors displayed on the image display means each time the color conversion characteristic candidate is set (initially set or altered), and the tristimulus values of colors displayed on the image display means need to be measured only when the characteristic of the image display means used for the prediction calculation is determined. Accordingly, the time required for the determination of the color conversion characteristic can be reduced substantially. Moreover, the color conversion characteristic is determined by comparing the predicted tristimulus values and the tristimulus values in the target color reproduction, it is possible to determine the color conversion characteristic realizing a color reproduction close to the ideal color reproduction according to sRGB, NTSC or like standard. For this reason, even when the operator does not have a clear idea of the image in the target color reproduction, it is possible to determine the color conversion characteristic.
Said step (d) of determining said predicted tristimulus values may include the steps of:
(d1) determining predicted image display data which are predicted values of the data input to said image display means, based on said predicted color-converted data; and
(d2) determining predicted tristimulus values which are predicted values of tristimulus values of colors displayed on said image display means, based on said predicted image display data.
With the above arrangement, it is possible to determine the color conversion characteristic even when the color-converted data output from the color conversion means and the image display data input to the image display means are different from each other.
Said predicted image display data and said predicted color-converted data may be identical to each other.
With the above arrangement, when the color-converted data output from the color conversion means and the image display data input to the image display means are identical to each other (for example, when the output of the color conversion means are supplied to the image display means without alteration) the color conversion characteristic can be determined.
Said step (e) of evaluating said predicted tristimulus values may include the steps of:
(e1) determining target tristimulus values which are tristimulus values in the target color reproduction characteristic, based on said input image data;
(e2) determining color differences between said predicted tristimulus values and said target tristimulus values; and
(e3) comparing the color differences between said predicted tristimulus values and said target tristimulus values as determined at said step (e2), with a predetermined reference value.
With the above arrangement, it is possible to evaluate the predicted tristimulus values, using the color differences. As a result, it is possible to determine the color conversion characteristic, without relying on the operator""s subjective evaluation (evaluation by means of the sense of sight).
Said tristimulus values may be tristimulus values according to XYZ color system.
With the above arrangement, it is possible to determine the color conversion characteristic using tristimulus values according to the XYZ color system which is widely used.
The color differences between said predicted tristimulus values and said target tristimulus values may be the color differences determined by the color difference formula according to the CIE1976 L*a*b* color system.
With the above arrangement, it is possible to evaluate the color differences using the color difference formula according to the CIE1976 L*a*b* color system representing a uniform color space, which is widely used for calculation of the color difference.
The color differences between said predicted tristimulus values and said target tristimulus values may be color differences determined by the CIE1994 color difference formula.
With the above arrangement, it is possible to evaluate the color differences using the CIE1994 color difference formula, the color differences determined thereby being generally coinciding closely in magnitude with those by the sense of sight.
Said step (d2) of calculating said predicted tristimulus values may include the steps of:
(d21) calculating predicted monochromatic tristimulus values which are predicted values of said tristimulus values of colors displayed on said image display means, based on each monochromatic component of said predicted image display data; and
(d22) calculating said predicted tristimulus values, using said predicted monochromatic tristimulus values.
With the above arrangement, it is possible to determine the predicted tristimulus values accurately.
Said step (d21) of calculating said predicted monochromatic tristimulus values may include calculating the predicted monochromatic tristimulus values according to a higher-order polynomial using said predicted image display data.
With the above arrangement, it is possible to calculate the predicted monochromatic tristimulus values accurately.
Said step of calculating said predicted monochromatic tristimulus values may include calculating the predicted monochromatic tristimulus values Xr, Yr, Zr, Xg, Yg, Zg, Xb, Yb, Zb, according to the following formulae (1) to (3), using said predicted image display data R1, G1, B1, and coefficients ARij, AGij, Abij (i=1 to 3, j=1 to n, n being an integer not smaller than 3).                               [                                                    Xr                                                                    Yr                                                                    Zr                                              ]                =                              (            ARij            )                    ⁡                      [                                                                                R1                    n                                                                                                                    R1                                          n                      -                      1                                                                                                                    ⋮                                                                                                  R1                    2                                                                                                R1                                                                              1                                                      ]                                              (        1        )                                          [                                                    Xg                                                                    Yg                                                                    Zg                                              ]                =                              (            AGij            )                    ⁡                      [                                                                                G1                    n                                                                                                                    G1                                          n                      -                      1                                                                                                                    ⋮                                                                                                  G1                    2                                                                                                G1                                                                              1                                                      ]                                              (        2        )                                          [                                                    Xb                                                                    Yb                                                                    Zb                                              ]                =                              (            ABij            )                    ⁡                      [                                                                                B1                    n                                                                                                                    B1                                          n                      -                      1                                                                                                                    ⋮                                                                                                  B1                    2                                                                                                B1                                                                              1                                                      ]                                              (        3        )            
With the above arrangement, it is possible to determine the predicted monochromatic tristimulus values accurately, using simple formulae consisting of multiplications and additions.
Said step (d22) of calculating said predicted tristimulus values from said predicted monochromatic tristimulus values includes calculating the predicted tristimulus values according to polynomials using said predicted monochromatic tristimulus values.
With the above arrangement, it is possible to calculate the predicted tristimulus values accurately using the predicted monochromatic tristimulus values.
Said step (d22) of calculating said predicted tristimulus values from said predicted monochromatic tristimulus values may include calculating the predicted tristimulus values X, Y, Z according to the following formula (4) using said predicted monochromatic tristimulus values Xr, Yr, Zr, Xg, Yg, Zg, Xb, Yb, Zb, and coefficients Eik (i=1 to 3, k=1 to 30).                               [                                                    X                                                                    Y                                                                    Z                                              ]                =                                            (              Eik              )                        ⁡                          [                                                                    Xr                                                                                        Xg                                                                                        Xb                                                                                                              Xr                      2                                                                                                                                  Xg                      2                                                                                                                                  Xb                      2                                                                                                                                  Xr                      xc3x97                      Xg                                                                                                                                  Xg                      xc3x97                      Xb                                                                                                                                  Xb                      xc3x97                      Xr                                                                                                                                  Xr                      xc3x97                      Xg                      xc3x97                      Xb                                                                                                            Yr                                                                                        Yg                                                                                        Yb                                                                                                              Yr                      2                                                                                                                                  Yg                      2                                                                                                                                  Yb                      2                                                                                                                                  Yr                      xc3x97                      Yg                                                                                                                                  Yg                      xc3x97                      Yb                                                                                                                                  Yb                      xc3x97                      Yr                                                                                                                                  Yr                      xc3x97                      Yg                      xc3x97                      Yb                                                                                                            Zr                                                                                        Zg                                                                                        Zb                                                                                                              Zr                      2                                                                                                                                  Zg                      2                                                                                                                                  Zb                      2                                                                                                                                  Zr                      xc3x97                      Zg                                                                                                                                  Zg                      xc3x97                      Zb                                                                                                                                  Zb                      xc3x97                      Zr                                                                                                                                  Zr                      xc3x97                      Zg                      xc3x97                      Zb                                                                                  ]                                +                      [                                                            Xbk                                                                              Ybk                                                                              Zbk                                                      ]                                              (        4        )            
With the above arrangeemnt, it is possible to calculate the predicted tristimulus values accurately using simple formulae consisting of multiplications and additions.
Said step (d22) of calculating said predicted tristimulus values from said predicted monochromatic tristimulus values includes calculating predicted tristimulus values X, Y, Z according to the following formula (5) using said predicted monochromatic tristimulus values Xr, Yr, Zr, Xg, Yg, Zg, Xb, Yb, Zb, and coefficients Eik (i=1 to 3, k=1 to 39).                               [                                                    X                                                                    Y                                                                    Z                                              ]                =                                            (              Eik              )                        ⁡                          [                                                                    Xr                                                                                        Xg                                                                                        Xb                                                                                                              Xr                      2                                                                                                                                  Xg                      2                                                                                                                                  Xb                      2                                                                                                                                  Xr                      xc3x97                      Xg                                                                                                                                  Xg                      xc3x97                      Xb                                                                                                                                  Xb                      xc3x97                      Xr                                                                                                                                                          Xr                        xc3x97                        Xg                                                                                                                                                                                Xg                        xc3x97                        Xb                                                                                                                                                                                Xb                        xc3x97                        Xr                                                                                                                                                        Xr                      xc3x97                      Xg                      xc3x97                      Xb                                                                                                            Yr                                                                                        Yg                                                                                        Yb                                                                                                              Yr                      2                                                                                                                                  Yg                      2                                                                                                                                  Yb                      2                                                                                                                                  Yr                      xc3x97                      Yg                                                                                                                                  Yg                      xc3x97                      Yb                                                                                                                                  Yb                      xc3x97                      Yr                                                                                                                                                          Yr                        xc3x97                        Yg                                                                                                                                                                                Yg                        xc3x97                        Yb                                                                                                                                                                                Yb                        xc3x97                        Yr                                                                                                                                                        Yr                      xc3x97                      Yg                      xc3x97                      Yb                                                                                                            Zr                                                                                        Zg                                                                                        Zb                                                                                                              Zr                      2                                                                                                                                  Zg                      2                                                                                                                                  Zb                      2                                                                                                                                  Zr                      xc3x97                      Zg                                                                                                                                  Zg                      xc3x97                      Zb                                                                                                                                  Zb                      xc3x97                      Zr                                                                                                                                                          Zr                        xc3x97                        Zg                                                                                                                                                                                Zg                        xc3x97                        Zb                                                                                                                                                                                Zb                        xc3x97                        Zr                                                                                                                                                        Zr                      xc3x97                      Zg                      xc3x97                      Zb                                                                                  ]                                +                      [                                                            Xbk                                                                              Ybk                                                                              Zbk                                                      ]                                              (        5        )            
With the above arrangement, it is possible to calculate the predicted tristimulus values more accurately using formulae including calculation of the square roots.
According another aspect of the invention, there is provided an image display device including a color conversion characteristic holding means (3) holding color conversion characteristic; a color conversion means (2) converting, on pixel-by-pixel basis, image data represented by three or more colors, referring to said color conversion characteristic; and an image display means (5) performing display of the image data obtained by color conversion at said color conversion means (2);
wherein color conversion characteristic determined in accordance with the above-described method are held in said color conversion characteristic holding means.
With the above arrangement, it is possible to reduce the time required for determining the color conversion characteristic substantially. Moreover, if the color conversion characteristic is determined by comparing the tristimulus values in the target color reproduction, and the predicted tristimulus values, it is possible to determine the color conversion characteristic realizing a color reproduction close to the ideal color reproduction according to the sRGB, NTSC or like standard. Moreover, it is possible to determine the color conversion characteristic even when the operator does not have a clear idea of the image in the target color reproduction.
According to a further aspect of the invention, there is provided a recording medium which can be read by a computer system, and which stores a computer program for causing a computer system to implement the above-described method.
With the above arrangement, it is possible to have a computer system implement the above-described method.